/**
 * $Revision: 1456 $
 * $Date: 2005-06-01 22:04:54 -0700 (Wed, 01 Jun 2005) $
 *
 * Copyright 2003-2005 Jive Software.
 *
 * All rights reserved. Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.jivesoftware.smack.util;

import org.jivesoftware.smack.util.collections.AbstractMapEntry;

import java.util.*;

/**
 * A specialized Map that is size-limited (using an LRU algorithm) and has an
 * optional expiration time for cache items. The Map is thread-safe.
 * <p>
 * 
 * The algorithm for cache is as follows: a HashMap is maintained for fast
 * object lookup. Two linked lists are maintained: one keeps objects in the
 * order they are accessed from cache, the other keeps objects in the order they
 * were originally added to cache. When objects are added to cache, they are
 * first wrapped by a CacheObject which maintains the following pieces of
 * information:
 * <ul>
 * <li>A pointer to the node in the linked list that maintains accessed order
 * for the object. Keeping a reference to the node lets us avoid linear scans of
 * the linked list.
 * <li>A pointer to the node in the linked list that maintains the age of the
 * object in cache. Keeping a reference to the node lets us avoid linear scans
 * of the linked list.
 * </ul>
 * <p/>
 * To get an object from cache, a hash lookup is performed to get a reference to
 * the CacheObject that wraps the real object we are looking for. The object is
 * subsequently moved to the front of the accessed linked list and any necessary
 * cache cleanups are performed. Cache deletion and expiration is performed as
 * needed.
 * 
 * @author Matt Tucker
 */
public class Cache<K, V> implements Map<K, V> {

	/**
	 * The map the keys and values are stored in.
	 */
	protected Map<K, CacheObject<V>> map;

	/**
	 * Linked list to maintain order that cache objects are accessed in, most
	 * used to least used.
	 */
	protected LinkedList lastAccessedList;

	/**
	 * Linked list to maintain time that cache objects were initially added to
	 * the cache, most recently added to oldest added.
	 */
	protected LinkedList ageList;

	/**
	 * Maximum number of items the cache will hold.
	 */
	protected int maxCacheSize;

	/**
	 * Maximum length of time objects can exist in cache before expiring.
	 */
	protected long maxLifetime;

	/**
	 * Maintain the number of cache hits and misses. A cache hit occurs every
	 * time the get method is called and the cache contains the requested
	 * object. A cache miss represents the opposite occurence.
	 * <p>
	 * 
	 * Keeping track of cache hits and misses lets one measure how efficient the
	 * cache is; the higher the percentage of hits, the more efficient.
	 */
	protected long cacheHits, cacheMisses = 0L;

	/**
	 * Create a new cache and specify the maximum size of for the cache in
	 * bytes, and the maximum lifetime of objects.
	 * 
	 * @param maxSize
	 *            the maximum number of objects the cache will hold. -1 means
	 *            the cache has no max size.
	 * @param maxLifetime
	 *            the maximum amount of time (in ms) objects can exist in cache
	 *            before being deleted. -1 means objects never expire.
	 */
	public Cache(int maxSize, long maxLifetime) {
		if (maxSize == 0) {
			throw new IllegalArgumentException("Max cache size cannot be 0.");
		}
		this.maxCacheSize = maxSize;
		this.maxLifetime = maxLifetime;

		// Our primary data structure is a hash map. The default capacity of 11
		// is too small in almost all cases, so we set it bigger.
		map = new HashMap<K, CacheObject<V>>(103);

		lastAccessedList = new LinkedList();
		ageList = new LinkedList();
	}

	public synchronized V put(K key, V value) {
		V oldValue = null;
		// Delete an old entry if it exists.
		if (map.containsKey(key)) {
			oldValue = remove(key, true);
		}

		CacheObject<V> cacheObject = new CacheObject<V>(value);
		map.put(key, cacheObject);
		// Make an entry into the cache order list.
		// Store the cache order list entry so that we can get back to it
		// during later lookups.
		cacheObject.lastAccessedListNode = lastAccessedList.addFirst(key);
		// Add the object to the age list
		LinkedListNode ageNode = ageList.addFirst(key);
		ageNode.timestamp = System.currentTimeMillis();
		cacheObject.ageListNode = ageNode;

		// If cache is too full, remove least used cache entries until it is not
		// too full.
		cullCache();

		return oldValue;
	}

	public synchronized V get(Object key) {
		// First, clear all entries that have been in cache longer than the
		// maximum defined age.
		deleteExpiredEntries();

		CacheObject<V> cacheObject = map.get(key);
		if (cacheObject == null) {
			// The object didn't exist in cache, so increment cache misses.
			cacheMisses++;
			return null;
		}
		// Remove the object from it's current place in the cache order list,
		// and re-insert it at the front of the list.
		cacheObject.lastAccessedListNode.remove();
		lastAccessedList.addFirst(cacheObject.lastAccessedListNode);

		// The object exists in cache, so increment cache hits. Also, increment
		// the object's read count.
		cacheHits++;
		cacheObject.readCount++;

		return cacheObject.object;
	}

	public synchronized V remove(Object key) {
		return remove(key, false);
	}

	/*
	 * Remove operation with a flag so we can tell coherence if the remove was
	 * caused by cache internal processing such as eviction or loading
	 */
	public synchronized V remove(Object key, boolean internal) {
		// noinspection SuspiciousMethodCalls
		CacheObject<V> cacheObject = map.remove(key);
		// If the object is not in cache, stop trying to remove it.
		if (cacheObject == null) {
			return null;
		}
		// Remove from the cache order list
		cacheObject.lastAccessedListNode.remove();
		cacheObject.ageListNode.remove();
		// Remove references to linked list nodes
		cacheObject.ageListNode = null;
		cacheObject.lastAccessedListNode = null;

		return cacheObject.object;
	}

	public synchronized void clear() {
		Object[] keys = map.keySet().toArray();
		for (Object key : keys) {
			remove(key);
		}

		// Now, reset all containers.
		map.clear();
		lastAccessedList.clear();
		ageList.clear();

		cacheHits = 0;
		cacheMisses = 0;
	}

	public synchronized int size() {
		// First, clear all entries that have been in cache longer than the
		// maximum defined age.
		deleteExpiredEntries();

		return map.size();
	}

	public synchronized boolean isEmpty() {
		// First, clear all entries that have been in cache longer than the
		// maximum defined age.
		deleteExpiredEntries();

		return map.isEmpty();
	}

	public synchronized Collection<V> values() {
		// First, clear all entries that have been in cache longer than the
		// maximum defined age.
		deleteExpiredEntries();

		return Collections.unmodifiableCollection(new AbstractCollection<V>() {
			Collection<CacheObject<V>> values = map.values();

			public Iterator<V> iterator() {
				return new Iterator<V>() {
					Iterator<CacheObject<V>> it = values.iterator();

					public boolean hasNext() {
						return it.hasNext();
					}

					public V next() {
						return it.next().object;
					}

					public void remove() {
						it.remove();
					}
				};
			}

			public int size() {
				return values.size();
			}
		});
	}

	public synchronized boolean containsKey(Object key) {
		// First, clear all entries that have been in cache longer than the
		// maximum defined age.
		deleteExpiredEntries();

		return map.containsKey(key);
	}

	public void putAll(Map<? extends K, ? extends V> map) {
		for (Entry<? extends K, ? extends V> entry : map.entrySet()) {
			V value = entry.getValue();
			// If the map is another DefaultCache instance than the
			// entry values will be CacheObject instances that need
			// to be converted to the normal object form.
			if (value instanceof CacheObject) {
				// noinspection unchecked
				value = ((CacheObject<V>) value).object;
			}
			put(entry.getKey(), value);
		}
	}

	public synchronized boolean containsValue(Object value) {
		// First, clear all entries that have been in cache longer than the
		// maximum defined age.
		deleteExpiredEntries();

		// noinspection unchecked
		CacheObject<V> cacheObject = new CacheObject<V>((V) value);

		return map.containsValue(cacheObject);
	}

	public synchronized Set<Map.Entry<K, V>> entrySet() {
		// Warning -- this method returns CacheObject instances and not Objects
		// in the same form they were put into cache.

		// First, clear all entries that have been in cache longer than the
		// maximum defined age.
		deleteExpiredEntries();

		return new AbstractSet<Map.Entry<K, V>>() {
			private final Set<Map.Entry<K, CacheObject<V>>> set = map
					.entrySet();

			public Iterator<Entry<K, V>> iterator() {
				return new Iterator<Entry<K, V>>() {
					private final Iterator<Entry<K, CacheObject<V>>> it = set
							.iterator();

					public boolean hasNext() {
						return it.hasNext();
					}

					public Entry<K, V> next() {
						Map.Entry<K, CacheObject<V>> entry = it.next();
						return new AbstractMapEntry<K, V>(entry.getKey(),
								entry.getValue().object) {
							@Override
							public V setValue(V value) {
								throw new UnsupportedOperationException(
										"Cannot set");
							}
						};
					}

					public void remove() {
						it.remove();
					}
				};

			}

			public int size() {
				return set.size();
			}
		};
	}

	public synchronized Set<K> keySet() {
		// First, clear all entries that have been in cache longer than the
		// maximum defined age.
		deleteExpiredEntries();

		return Collections.unmodifiableSet(map.keySet());
	}

	public long getCacheHits() {
		return cacheHits;
	}

	public long getCacheMisses() {
		return cacheMisses;
	}

	public int getMaxCacheSize() {
		return maxCacheSize;
	}

	public synchronized void setMaxCacheSize(int maxCacheSize) {
		this.maxCacheSize = maxCacheSize;
		// It's possible that the new max size is smaller than our current cache
		// size. If so, we need to delete infrequently used items.
		cullCache();
	}

	public long getMaxLifetime() {
		return maxLifetime;
	}

	public void setMaxLifetime(long maxLifetime) {
		this.maxLifetime = maxLifetime;
	}

	/**
	 * Clears all entries out of cache where the entries are older than the
	 * maximum defined age.
	 */
	protected synchronized void deleteExpiredEntries() {
		// Check if expiration is turned on.
		if (maxLifetime <= 0) {
			return;
		}

		// Remove all old entries. To do this, we remove objects from the end
		// of the linked list until they are no longer too old. We get to avoid
		// any hash lookups or looking at any more objects than is strictly
		// neccessary.
		LinkedListNode node = ageList.getLast();
		// If there are no entries in the age list, return.
		if (node == null) {
			return;
		}

		// Determine the expireTime, which is the moment in time that elements
		// should expire from cache. Then, we can do an easy check to see
		// if the expire time is greater than the expire time.
		long expireTime = System.currentTimeMillis() - maxLifetime;

		while (expireTime > node.timestamp) {
			if (remove(node.object, true) == null) {
				System.err.println("Error attempting to remove("
						+ node.object.toString()
						+ ") - cacheObject not found in cache!");
				// remove from the ageList
				node.remove();
			}

			// Get the next node.
			node = ageList.getLast();
			// If there are no more entries in the age list, return.
			if (node == null) {
				return;
			}
		}
	}

	/**
	 * Removes the least recently used elements if the cache size is greater
	 * than or equal to the maximum allowed size until the cache is at least 10%
	 * empty.
	 */
	protected synchronized void cullCache() {
		// Check if a max cache size is defined.
		if (maxCacheSize < 0) {
			return;
		}

		// See if the cache is too big. If so, clean out cache until it's 10%
		// free.
		if (map.size() > maxCacheSize) {
			// First, delete any old entries to see how much memory that frees.
			deleteExpiredEntries();
			// Next, delete the least recently used elements until 10% of the
			// cache
			// has been freed.
			int desiredSize = (int) (maxCacheSize * .90);
			for (int i = map.size(); i > desiredSize; i--) {
				// Get the key and invoke the remove method on it.
				if (remove(lastAccessedList.getLast().object, true) == null) {
					System.err
							.println("Error attempting to cullCache with remove("
									+ lastAccessedList.getLast().object
											.toString()
									+ ") - "
									+ "cacheObject not found in cache!");
					lastAccessedList.getLast().remove();
				}
			}
		}
	}

	/**
	 * Wrapper for all objects put into cache. It's primary purpose is to
	 * maintain references to the linked lists that maintain the creation time
	 * of the object and the ordering of the most used objects.
	 * 
	 * This class is optimized for speed rather than strictly correct
	 * encapsulation.
	 */
	private static class CacheObject<V> {

		/**
		 * Underlying object wrapped by the CacheObject.
		 */
		public V object;

		/**
		 * A reference to the node in the cache order list. We keep the
		 * reference here to avoid linear scans of the list. Every time the
		 * object is accessed, the node is removed from its current spot in the
		 * list and moved to the front.
		 */
		public LinkedListNode lastAccessedListNode;

		/**
		 * A reference to the node in the age order list. We keep the reference
		 * here to avoid linear scans of the list. The reference is used if the
		 * object has to be deleted from the list.
		 */
		public LinkedListNode ageListNode;

		/**
		 * A count of the number of times the object has been read from cache.
		 */
		public int readCount = 0;

		/**
		 * Creates a new cache object wrapper.
		 * 
		 * @param object
		 *            the underlying Object to wrap.
		 */
		public CacheObject(V object) {
			this.object = object;
		}

		public boolean equals(Object o) {
			if (this == o) {
				return true;
			}
			if (!(o instanceof CacheObject)) {
				return false;
			}

			final CacheObject<?> cacheObject = (CacheObject<?>) o;

			return object.equals(cacheObject.object);

		}

		public int hashCode() {
			return object.hashCode();
		}
	}

	/**
	 * Simple LinkedList implementation. The main feature is that list nodes are
	 * public, which allows very fast delete operations when one has a reference
	 * to the node that is to be deleted.
	 * <p>
	 */
	private static class LinkedList {

		/**
		 * The root of the list keeps a reference to both the first and last
		 * elements of the list.
		 */
		private LinkedListNode head = new LinkedListNode("head", null, null);

		/**
		 * Creates a new linked list.
		 */
		public LinkedList() {
			head.next = head.previous = head;
		}

		/**
		 * Returns the first linked list node in the list.
		 * 
		 * @return the first element of the list.
		 */
		public LinkedListNode getFirst() {
			LinkedListNode node = head.next;
			if (node == head) {
				return null;
			}
			return node;
		}

		/**
		 * Returns the last linked list node in the list.
		 * 
		 * @return the last element of the list.
		 */
		public LinkedListNode getLast() {
			LinkedListNode node = head.previous;
			if (node == head) {
				return null;
			}
			return node;
		}

		/**
		 * Adds a node to the beginning of the list.
		 * 
		 * @param node
		 *            the node to add to the beginning of the list.
		 * @return the node
		 */
		public LinkedListNode addFirst(LinkedListNode node) {
			node.next = head.next;
			node.previous = head;
			node.previous.next = node;
			node.next.previous = node;
			return node;
		}

		/**
		 * Adds an object to the beginning of the list by automatically creating
		 * a a new node and adding it to the beginning of the list.
		 * 
		 * @param object
		 *            the object to add to the beginning of the list.
		 * @return the node created to wrap the object.
		 */
		public LinkedListNode addFirst(Object object) {
			LinkedListNode node = new LinkedListNode(object, head.next, head);
			node.previous.next = node;
			node.next.previous = node;
			return node;
		}

		/**
		 * Adds an object to the end of the list by automatically creating a a
		 * new node and adding it to the end of the list.
		 * 
		 * @param object
		 *            the object to add to the end of the list.
		 * @return the node created to wrap the object.
		 */
		public LinkedListNode addLast(Object object) {
			LinkedListNode node = new LinkedListNode(object, head,
					head.previous);
			node.previous.next = node;
			node.next.previous = node;
			return node;
		}

		/**
		 * Erases all elements in the list and re-initializes it.
		 */
		public void clear() {
			// Remove all references in the list.
			LinkedListNode node = getLast();
			while (node != null) {
				node.remove();
				node = getLast();
			}

			// Re-initialize.
			head.next = head.previous = head;
		}

		/**
		 * Returns a String representation of the linked list with a comma
		 * delimited list of all the elements in the list.
		 * 
		 * @return a String representation of the LinkedList.
		 */
		public String toString() {
			LinkedListNode node = head.next;
			StringBuilder buf = new StringBuilder();
			while (node != head) {
				buf.append(node.toString()).append(", ");
				node = node.next;
			}
			return buf.toString();
		}
	}

	/**
	 * Doubly linked node in a LinkedList. Most LinkedList implementations keep
	 * the equivalent of this class private. We make it public so that
	 * references to each node in the list can be maintained externally.
	 * 
	 * Exposing this class lets us make remove operations very fast. Remove is
	 * built into this class and only requires two reference reassignments. If
	 * remove existed in the main LinkedList class, a linear scan would have to
	 * be performed to find the correct node to delete.
	 * 
	 * The linked list implementation was specifically written for the Jive
	 * cache system. While it can be used as a general purpose linked list, for
	 * most applications, it is more suitable to use the linked list that is
	 * part of the Java Collections package.
	 */
	private static class LinkedListNode {

		public LinkedListNode previous;
		public LinkedListNode next;
		public Object object;

		/**
		 * This class is further customized for the Jive cache system. It
		 * maintains a timestamp of when a Cacheable object was first added to
		 * cache. Timestamps are stored as long values and represent the number
		 * of milliseconds passed since January 1, 1970 00:00:00.000 GMT.
		 * <p>
		 * 
		 * The creation timestamp is used in the case that the cache has a
		 * maximum lifetime set. In that case, when [current time] - [creation
		 * time] > [max lifetime], the object will be deleted from cache.
		 */
		public long timestamp;

		/**
		 * Constructs a new linked list node.
		 * 
		 * @param object
		 *            the Object that the node represents.
		 * @param next
		 *            a reference to the next LinkedListNode in the list.
		 * @param previous
		 *            a reference to the previous LinkedListNode in the list.
		 */
		public LinkedListNode(Object object, LinkedListNode next,
				LinkedListNode previous) {
			this.object = object;
			this.next = next;
			this.previous = previous;
		}

		/**
		 * Removes this node from the linked list that it is a part of.
		 */
		public void remove() {
			previous.next = next;
			next.previous = previous;
		}

		/**
		 * Returns a String representation of the linked list node by calling
		 * the toString method of the node's object.
		 * 
		 * @return a String representation of the LinkedListNode.
		 */
		public String toString() {
			return object.toString();
		}
	}
}